In each of LTE-A (Long Term Evolution Advanced) standardized in the standards body 3GPP (The 3rd Generation Partnership Project) and IEEE 802.16m (next generation WiMAX) standardized in the IEEE 802.16 Committee, the MU-MIMO (MultiUser-MIMO) system has been considered as the MIMO (Multi Input Multi Output) transmission system of a downlink. The MU-MIMO system is a system where a base station having a plurality of antennas communicates with a plurality of communications terminals (hereinafter merely referred to “terminals”) having a plurality of antennas. According to this system, the base station distributes resource to the terminals and transmits different signals to the terminals at respective directivities.
The base station performs the channel adaptive scheduling. The base station performing the channel adaptive scheduling distributes basically the resource in the time direction and the frequency direction in a block unit basis to the terminals in a good channel state. Further, the base station employing the MU-MIMO system also distributes the resource in the spatial direction in addition to the distribution of the resource in the time direction and the frequency direction.
The sizes of data transmitted to the terminals from the base station may differ at the respective terminals. Further, as described above, according to the channel adaptive scheduling, the terminal is selected in accordance with the channel state thereof. Thus, the sizes of the data transmitted to the terminals, each of which is allowed to communicate, do not necessarily coincide to each other. As a result, the resource distributed to the terminal having a small data size contains empty resource blocks each having no actual data.
FIG. 12 is a diagram showing an example of data, distributed to logical resource blocks in the frequency direction, to be transmitted to two terminals from the base station employing the MU-MIMO system. In the example shown in FIG. 12, four VRBs (Virtual Resource Blocks) are allocated to each of the terminal #1 and the terminal #2. VBR is a unit of data distribution in a state before the data distribution as described later. In this example, although the data size of the data to be transmitted to the terminal #1 corresponds to four VBRs, the data size of the data to be transmitted to the terminal #2 corresponds to two VBRs. In this manner, since two VBRs among the four VBRs distributed to the terminal #2 contain no actual data, there arises empty resource blocks within the resource. In this specification, it is supposed that data to be transmitted to terminals other than the terminals #1 and #2 is not distributed to the empty resource blocks.
FIGS. 13(a) and (b) are diagrams showing an example of data, to be transmitted to the respective terminals, each distributed to PRBs (Physical Resource Blocks) in the time direction and the frequency direction. FIG. 13(a) shows the data to be transmitted to the terminal #1 and FIG. 13(b) shows the data to be transmitted to the terminal #2. As shown in FIGS. 13(a) and (b), the data distributed to the respective VRBs of each of the respective terminals is disposed on the corresponding PRBs in one-to-one correspondence with the VRBs in the frequency direction at every terminal. PRB is a unit of data distribution in a state after the data distribution as described later, and configured by a plurality of resource blocks in the time direction and the frequency direction. For example, as shown in FIG. 13(b), the PRB is configured by 18 subcarriers in the frequency direction and 6 symbols in the time direction. Further, when the data is distributed on the PRBs, the pilot symbols are inserted at different positions for the respective terminals.
In the wireless mobile communications, the signal waveform is distorted due to the influence of the multipath fading on a propagation channel (hereinafter merely referred to “channel”). The signal transmitted to the terminals from the base station is also distorted. In order for the terminal to correctly decode the signal transmitted from the base station, the terminal is required to estimate the channel and compensate the signal. For the channel estimation, the base station transmits the pilot symbols known for both the base station and the terminals. A plurality of the pilot symbols are disposed within each of the PRBs at a constant interval in the time direction or the frequency direction.
FIG. 14 is a diagram showing an example of multiplexed data of the data for the respective terminals shown in FIGS. 13(a) and (b). As shown in FIG. 14, the pilot symbols of the respective terminals to be multiplexed are disposed at the constant interval in the time direction or the frequency direction. The channel estimation is a processing in which a channel variation degree estimated by using the pilot symbols is subjected to an interpolation processing such as a weighting or averaging processing, and the channel of the data existing between the pilot symbols is estimated at every resource element (Resource Element: RE) shown in FIG. 14.
The technique disclosed in a patent literature 1 does not employ the MIMO system and is based on the communications with a single terminal. Further, the patent literature 1 has no disclosure relating to the empty resource blocks.